The ramjet principal of propulsion is well known in the art. During the flight of a ramjet powered vehicle, high velocity air enters a diffuser in the front of a ramjet engine which is shaped to slow the flowing air, thereby inducing compression of the airstream. The compression of the airstream generates a normal shock wave which slows the flowing air to subsonic velocities. As the air enters a combustion chamber, fuel is continuously injected into the combustion chamber and ignited, producing hot combustion gases. Forward vehicle thrust is provided by the ejection of the hot combustion gases through a discharge nozzle at a velocity greater than the flight speed. Since a ramjet relies on high air flow velocity through a diffuser rather than mechanical apparatus to achieve compression, ramjets require minimum flight speeds of approximately Mach 1-3 for efficient operation. Generally, chemical rocket motors or turbine type engines must be used to propel a ramjet-powered vehicle to such minimal flight speeds before ramjet propulsion is initiated.
Adapting the ramjet principal of propulsion to gun-fired projectiles significantly increases the range of artillery and the destructive potential of projectile discharging weapons. Conventional explosive propulsion generally accelerates a projectile to supersonic speeds between Mach 1.5-4.0. Ramjet propulsion extends the flight of a projectile by further accelerating such a projectile to hypersonic speeds (Mach 5.0 and above). Prior art weapons, utilizing the ramjet principle to boost projectile speed, have included various modified projectiles incorporating ramjet engines which initiate further acceleration after discharge from a conventional gun barrel. Such projectiles include an outer casing, an inner compression and combustion chamber, an integral fuel supply, and a discharge nozzle. U.S. Pat. No. 4,428,293 to Botwin et al discloses such a projectile which also includes variable thrust control of the projectile after discharge from a gun.
A ram cannon uses the ramjet principle to promote projectile acceleration before discharge from a gun barrel. By firing a projectile through a barrel section containing a fuel-oxidizer mixture, the projectile and barrel, in effect, become a ramjet engine with the barrel effectively forming the outer engine casing and the spacing between the projectile and barrel wall defining the compression and combustion chambers. In a subsonic combustion ram cannon (see FIG. 2a), a discharge nozzle is included which is defined by the annular spacing between the projectile tail and the barrel wall. As the projectile passes through the barrel, the premixed fuel-oxidizer mixture is compressed and ignited, generating hot combustion gases which expand rearwardly through the discharge nozzle, imparting forward thrust to the projectile.
A particular problem with subsonic combustion ram cannons is that such ramjet propulsion of a projectile within a gun barrel generates a rapid pressure build up during the projectile acceleration. A normal shock wave slows the flowing gas to subsonic velocities prior to combustion and induces a high pressure gradient directed to the barrel wall. It is at this point in the ramjet cycle that the peak pressure is encountered. Since the ram cannon design is limited by the barrel working pressure, a subsonic combustion ram cannon must be designed for the shock pressure. Consequently, the maximum muzzle velocity of the projectile is limited by the pressure rating of the barrel relative to the high pressure spike that occurs at the point of normal shock.
Another problem with subsonic combustion ram cannons involves the possibility of propagating a detonation wave ahead of the moving projectile into the unburned fuel-oxidizer mixture, resulting in a preignition of the fuel-oxidizer mixture, halting acceleration of the projectile.
Several alternatives have been proposed for alleviating this problem. Utilizing either a smaller diameter projectile or an oversized bore would increase the spacing between the barrel wall and projectile body, thereby decreasing the amount of fuel-oxidizer compression and moderating the normal shock pressure. However, such a loss in propulsion efficiency would also limit the projectile acceleration, thereby requiring a longer barrel to achieve a hypersonic muzzle velocity. Another proposed solution involves increasing the barrel working pressure by such methods as increasing barrel strength through increased wall thickness. However, while some weapons could incorporate such strengthened barrels, the costs and weights involved would be prohibitive.
Another alternative, disclosed in commonly assigned U.S. patent application Ser. No. 857,687 to Titus, titled "Ram Cannon Barrel", filed Apr. 31, 1986, involves the use of an outwardly flared barrel bore which provides added bore volume to offset the pressure increases. While useful in moderating the pressure buildup within the barrel, a major structural modification of the cannon barrel is required, and the maximum projectile acceleration is still structurally limited.
A variation of the subsonic combustion ram cannon utilizes a thermally choked combustion cycle (see FIG. 2b). In this cycle, the combustion takes place behind the projectile in the full barrel bore area. The combustion process therefore reaccelerates the gas flow to supersonic speed in the aft barrel area, thereby accelerating the projectile. While providing good performance at low speeds, the thrust drops off dramatically when the projectile approaches the detonation wave velocity of the propellant fuel-oxidizer mixture.
Utilizing supersonic combustion (see FIG. 2c) in a ram cannon has been investigated as a method of avoiding a normal shock and the concomitant high pressure peak. However, such supersonic combustion ram cannons include a tail section which confines the combustion area, leading to the build up of high pressure gradients in the combustion zone. Eventually, at high velocity, the supersonic combustion zone will narrow until an oblique detonation wave forms (see FIG. 2d), providing a very narrow reaction zone, similar to the normal shock wave. Since this pressure cannot exceed the barrel limiting pressure, the high pressures generated with the oblique detonation wave effectively limits the potential thrust.
Consequently, the search continues for a ram cannon capable of attaining high muzzle velocities with optimum propulsion efficiency and forward thrust, maximizing projectile acceleration.